This invention relates to a radiometer, and more particularly to a method and apparatus for dual band reflectance radiometry, and still more particularly to a method and apparatus for ratioing narrow bands selected to provide positive identification of reflecting materials by at least one ratio of the two prominent spectral peaks characterizing the material.
Reflectance radiometers have been successfully used in detecting various minerals--from alunite to zeolite. For example, a portable reflectance spectrometer is disclosed by Dr. Alexander F. H. Goetz, one of the present inventors, and others, in U.S. Pat. No. 4,043,668. Briefly the spectrometer disclosed there included an optical unit and a digital recording unit for recording the intensity of reflected radiation over a spectral range (0.4 to 2.5 micrometers) selected by a filter wheel. The recorded spectral data are later analyzed to determine the composition of the materials which produce the reflectance radiation spectral data. This development of a portable reflectance spectrometer followed the development in 1967-1970 of a Multispectral Photography Experiment S-158 included in the APOLLO 12 mission. That experiment utilized multispectral imaging systems with analysis capability for determination of lunar lithographic boundaries remotely from orbit, but without real-time spectral data analysis. For a description of the experiment see Alexander F. H. Goetz, et al., "Apollo 12 Multispectral Photography Experiment," Geochimica Acta, Vol. 3, 2301-2310, MIT Press, 1971.
Following that development, new research programs were established in 1970-1973 to improve the accuracy of telescopic spectroradiometric imaging systems. The role of computer image processing in orbital multispectral photography was established as a means of enhancement. The first preliminary geologic investigations were undertaken in the field on the Colorado plateau to evaluate and interpret earth satellite (ERTS-1)) multispectral data, suitably enhanced. Studies were also carried out to determine the quality and use of ERTS radiometric information with reference to arid desert regions. See Alexander F. H. Goetz, et al., "Symposium." Mar. 5-9, 1973 NASA SP-327 at pages 403 to 411, and 1159 to 1167. Also Proceedings of the 4th Annual Conference on Remote Sensing in Arid Lands, 136-147, Univ. of Arizona, Tucson, Nov. 1973. The image enhancement technology used in this effort is discussed in a NASA New Technology Report, NPO-10541.
An earth applications effort was formally organized at the Jet Propulsion Laboratory (JPL) of the California Institute of Technology. A novel portable reflectance spectrometer was developed for the 0.4 to 2.5 micrometer range, also with digital recording in the field. This instrument is the subject of the aforesaid U.S. Pat. No. 4,043,668, assigned to California Institute of Technology. The electronic recording unit was a separate "backpack" system, with an inherent time delay prior to actual mineral identification. The unit did not incorporate features of the present invention, and had no real time readout capability for analysis, but was capable of recording about 200 spectra per day on compact digital tape cassettes using two persons in the field.
Increased activity from 1975-1978 in the field of multispectral imaging and analysis systems at JPL led to the development of systems with CCD imaging devices, readily interfaced with more rapid computer analysis and readout systems, as is more fully discussed in U.S. Pat. No. 4,134,683, by Alexander F. H. Goetz, et al. An imaging system including several arrays of charge coupled devices (CCD), or linear detector arrays, were used to obtain simultaneously spectral reflectance data at different wavelengths for a target area using a plurality of filters, each accommodating a particular bandwidth. Data from the arrays are recorded and later read out in parallel to a computer or microprocessor.
The microprocesor made it possible to analyze image data in real time, and to display the information superimposed on an image of one to provide geographic locations. However, generally speaking, fairly broad visible and near-IR bands were covered and only rough qualitative analysis of minerals or oil spill zones was possible. The system was not portable and could be programmed to look for only one specific material at a time.
The instrument of U.S. Pat. No. 4,134,683 included "band ratioing" using divider circuits. "Band ratioing" is a technique which seeks to provide positive identification of materials by measurement or calculation of ratios of the two most prominent spectral peaks, rather than a single peak, characterizing the material. Band ratioing thus creates ratios of two filtered channels to cancel out topographic effects, etc. Band ratioing is also helpful in dealing with the problem of high data correlation between channels caused by systematic effects such as topography.
The prior art discussed above provided a portable reflectance radiometer without real-time data processing, or a nonportable multichannel reflectance imaging radiometer with limited real-time data processing. There has been no reflectance radiometer with instantaneous, real-time data processing, nor has there been a portable instrument with the capability of monitoring different peaks continuously without obtaining a full spectrum. And finally there has been no portable instrument provided to monitor a selected few narrow band peaks simultaneously and, through instantaneous band ratioing of one band channel versus the second band channel for at least one ratio, identifying the nature of the material reflecting the radiation. Yet the experience with the above-cited prior art devices, when applied to mineral identification problems in the field, makes it clear that for rock or mineral classes, such as carbonates and silicates, instantaneous identification would be possible if a portable unit could provide instant band ratioing. A new kind of instrument has therefore been needed to give useful information directly at the sensor and in real time for instantaneous rock identification. It is the intention of the present invention to provide such a new kind of instrument.